Electric drive



E. F, w. ALEXANDERSON ELECTRIC DRIVE Filed April 1, 1942 April 6, 1943.

DUPLICATE APPARATUS DUPLICATE APPARATUS AUTOMATIC Inventof Ernst F WAlexanderson,

His Attorney.

Patented Apr. 6, 1943 ELECTRIC DRIVE ErnstF. W. Alexanderson,Schenectady, N. Y., assignor to General Electric Company, a corporationof New York Application April 1, 1942, Serial No. 437,238

6 Claims.

My invention relates. to electric drives and more particularly toimprovements in electric drives for ship propulsion.

The electric generating portion of electric drives for ship propulsioncomprises, in some instances, a plurality of parallel synchronousgenerators, each of which is driven by its own 'individual prime mover,such, for example, as

a Diesel engine. Heretofore, it had been proposed that each prime moverin such an electric drive be regulated in'response to the speed thereofby means of centrifugally operated speed governors sothat as the loadvaries, the speed governors control the respective prime movers tomaintain the speeds thereof at desired values. Means are also providedfor theoperator to adjust the setting of the governors so that thespeed, which they maintain constant, may be readily varied. Suitablemeans are also provided in such drives for automatically synchronizing agenerator with the other generators in case it falls out of step and formaintaining the proper division of load between the generators when theyare operatingin synchronism. Such a control for electric drives, whichmay be properly called a speed control arrangement, is disclosed andclaimed in my copending application, Serial No. 366,322, filed June 6,1940, and assigned to the same assignee asthis application.

In a speed control arrangement of the abovementioned type, it isdesirable, in order to obtain efficient operation, that suitable meansbe provided for controlling the excitationof each larly adapted to shippropulsion, which is not as complicated as the above-mentioned combinedspeed control and stability-regulating arrangement, but vvhich has allof the advantages.

thereof.

My improved electric drive may be characterized by saying that it is atorque control arrangement rather than a speed control arrange ment. Itis analogous to the control of an automobile in uhich the torque isprimarily varied by varying the supply of fuel to the engine, and thedesired speed is attained as a secondary effect. For example. when anautomobile is going up a hill, the engine has to develop more torque Iin order to maintain a given speed. Similarly, in a ship propulsionsystem, more torque has to be developed by the prime mover in order tomaintain a given speed under varying load conditions, as is the casewhen there is a high sea or when the ships course is changed.

In accordance with my invention, I control the torque of a prime moverdriving a synchronous generator so that the torque of the prime mover isautomatically adjusted to maintain a predetermined relation between theactual output of the generator and the maximum output obtainable fromthe generator with the same field excitation. By automaticallycontrolling the torque of a prime mover inthis manner, an operator canmaintain a constant speed of the elec tric drive under varying loadconditions or can adjust the speed to any desired value under constantor varying load conditions merely by controlling'the excitation ofthesynchronous machines.

There are disclosed in the prior art various types of stabilityindicating or torque margin relays which respond to predeterminedelectric conditions of a synchronous machine so that the position of therelay is an indication of the relationship existing between the actualoutput of the machine and the maximum output or pullout torqueobtainable therefrom with the same field excitation. The aforesaidLetters Patent; No. 1,461,571 discloses and claims such a relay per seand its use in connection with an arrangement for controlling theexcitation of a synchronous machine so as to maintain a desiredrelationship between the actual and pull-out torques of the machine. Inmy present invention, however, a torque margin relay is used to controldirectly the torque of the prime mover instead of the field excitationof the synchronous machine which, in my present arrangement, iscontrolled directly by the operator. My invention will be betterunderstood from; the following description when taken in connec-l tionwith the accompanying drawing, the single figure of whichdiagrammatically illustrates an electric drive for ship propulsionembodying my invention, and the scope of my invention will be pointedout in the appended claims.

In the electric drive shown in the drawing, l represents a mainpolyphase power circuit across which are connected a plurality ofsynchronous generators 2 and a synchronous motor 3. A re vcrsing switch4 is provided for controlling the connections between the motor 3 andthe circuit I. In order to simplify the drawing and to avoid mereduplication, I have diagrammatically shown enclosed in a rectangle oneof the generators 2 and its driving and individual control apparatus indetail, and have represented the other generators 2 and their respectivedriving and control apparatus by similar rectangles labeled duplicateapparatus."

Each of the synchronous generators 2 is provided with a direct currentfield winding 5, which is energized from a suitable source of directcurrent, such, for example, as a common exciter Ii. A suitable switch Iis provided for selectively controlling the connection of all of thefield windings 5 to the exciter G. The synchronous motor 3 is alsoprovided with a direct current winding 8, which is arranged to beconnected to the exciter- 6 by a suitable switch 9. An adjustablerheostat I is provided in the shunt field circuit of the exciter 8 forvarying the voltage thereof.

The generators 2 are separately driven by suitable prime movers I I,such, for example, as Diesel engines. Each prime mover II has a throttleI2, which is controlled by a separate reversible motor I3, shown as adirect current motor.

Each throttle controlling motor I3 is individually controlled by atorque margin relay ll, of any suitable type, which is so connected thatits position is an indication of the relationship existing between theactual output of the generator and the maximum output obtainabletherefrom with the same fluid excitation without the generator fallingout of step. In the particular embodiment shown in the drawing, I havedisclosed a torque margin 'relay 0! the type disclosed and claimed inthe aforesaid Letters Patent No. 1,461,561. This relay is a differentialrelay having two opposing coils I5 and I6 respectively energized inresponse to the voltage and current of the associated generator 2 sothat the relay responds to a predetermined ratio between the current andvoltage and independently of any changes in the frequency of theassociated generator. This result is accomplished by making the currentresponsive element I6 operate directly in response to variations in thecurrent and by making the voltage responsive element I5 operate directlyin response to the voltage and inversely in response to the frequency byconnecting a large inductance II in series with the winding of thevoltage element. Suitable means, such as a resistor I8 in the circuit ofeach of the voltage elements I5, may be provided for varying the tacts20, which are closed to complete a circuit for the reversible motor I3so that it rotates in the opposite direction and effects an adjustmentof the throttle I2 so as to increase the torque of the associated primemover I I when the torque of the voltage element I5 is less than thetorque of the current element I5.

Any suitable means, examples of which are well known in the art, may beprovided for automatically controlling each throttle I2 when theassociated generator 2 is operating asynchronously with respect to theother generators of the system so as to bring the asynchronouslyoperating generator back into synchronism. In my copending application,Serial No. 437,239, filed April 1, 1942, and assigned to the sameassignee as this application, I have disclosed and claimed an automaticsynchronizing arrangement which is particularly adapted for use with anelectric drive embodying my present invention. However, since thedetails of the automatic synchronizing arrangement form no part of mypresent invention, I have represented such automatic synchronizing meansmerely by a rectangle 2I containing the contacts 22, which are closedwhen the associated generator 2 is in synchronism with the othergenerators, the contacts 24 which are closed to complete a circuit forthe associated throttle controlling motor I3 so that it effects adecrease in the torque of the associated prime mover II when thegenerator is operating supersynchronously with respect to the othergenerators, and the contacts 25 which are closed to complete a circuitfor the associated throttle controlling motor I3 so that it effects anincrease in the torque of the associated prime mover II when thegenerator is operating subsynchronously with respect to the othergenerators. The contacts 22 are connected in series with the contacts I9and 20 of the associated torque margin relay I4 so that this relay cancontrol the operation of the associated prime mover I I only when theassociated generator is operating in synchronism with the othergenerators.

The operation of the embodiment of my in vention shown in the drawingmay be understood from the following consideration. With any given motorload and a given adjustment of the exciter field rheostat Ill and of therheostats I8, each torque margin relay I4 controls its associated primemover II so as to maintain a predetermined relationship, dependent uponthe adjustment of the associated rheostat II, between the voltage andcurrent of the associated generator 2. As is well known to those skilledin the art, this predetermined relationship between the generatorvoltage and current corresponds to a similar relationship between theactual torque and the maximum torque that the associated generator candevelop with the same field excitation without pulling out of step.

Any increase in the motor load, while the adjustments of the rheostatsI0 and I8 remain fixed, results in a momentary decrease in the speed ofeach prime mover II and its associated generator 2 so that voltage ofeach generator 2 decreases relative to the current output thereof.

This change in the relative values of the voltage and the current outputof each generator 2 causes the contacts 20 of the associated torquemargin relay I4 to close and complete a circuit for the associatedthrottle controlling motor I3 to effect an increase in the torque andspeed of the associated prime mover II and generator 2. Since theimpedance of the motor 3 increases substantially in proportion to thefrequency of the .current supplied thereto, the increase in speedbrought about by the operation of the asS0 ciated torque margin relay I4results in an increase in the generator voltage without; a materialchange in the generator current. Consequently, as soon as the generatorspeed has increased to a sufiicient value to cause the torque producedby the voltage winding I5 of the associated torque margin relay tobalance the torque produced by the current winding I6, the relay opensits contacts 20 and prevents any further the associated torque marginrelay H to close its contacts i9 and effect a decrease in thetorque vand speed of the associated prime mover H and generator 2 until thevoltage of the generator has been reduced to a suilicient value to causethe associated relay M to open its contacts l9.

Whenever the operator desires to change the speed of the electric drive,he merely has to adjust the exciter field rheostat ii) in such a manneras to increase the exciter voltage when he wants to increase the speedof the drive and to decrease the exciter voltage when he wants todecrease the speed of the drive.

When the operator effects an increase in the eXciter voltage, theelectric torque of each generator 2 is increased so that a momentarydecrease in the speedof the associated prime mover ii occurs whichcauses the voltage of each generator to decrease relative to the currentoutput thereof. This change in the relative values of the voltage andcurrent of each generator causes the contacts 20 of the associatedtorque margin relay id to close and eiiect, in the manner heretoforedescribed, an increase in the torque and speed of the associated primemover until a speed is reached when the torques of the prime mover andthe associated generator 2 are equal, at which time voltage and currentof the generator are such as to cause the associated torque margin relayM to open its contacts 20.

Similarly when the operator adjusts the rheostat Ill so as to effect adecrease in the exciter voltage, the electric torque of each generator 2is decreased so that a momentary increase in the speed of the associatedprime mover ii occurs which causes the voltage of each generator to,increase relative to the current output thereof. This change in therelative values of the voltage and current of each generator causesthe'contacts IQ of the associated torque margin relay M to close andeffect in the manner heretofore described a decrease in the torque andspeed of the associated prime mover until a, speed is reached when thetorques of the prime mover and the associated generator are equal, atwhich time the voltage and current of each generator are such as tocause the associated torque margin relay M to open its contacts i9.

It will be apparent that the margin between the actual torque and themaximum torque to which the relays i4 respond may be readily varied byadjusting the rheostats I8.

In the event that any generator 2 for any reason falls out of step withthe rest of the system, the associated automatic synchronizer 2ioperates in a manner well known in the art to close its contacts 24 andeffect a decrease in the speed of the associated prime mover Ii when thegenerator is operating supersynchronously and to closeits contacts 25and effect an increase in the speed of the associated prime mover iiwhen the generator is operating subsynchronously. Since the contacts 22are closed only when the associated generator 2 is in synchronisin withthe existing between the actual control the associated generator 2 whichis operwith respect to the other is rendered inoperative to speed of theassociated ating asynchronously generators. l

With an electric ship propulsion control of this type, the operator, canmaneuver the ship without any concern of the synchronism between themotor, and the generators or of the synchronism between the differentgenerators. The proper division of load between the several generatorsoccurs automatically, and the operator regulates the torque of theelectric drive entirely by controlling the excitation ofthe exciter.

While I have, in accordance with the patent statutes. applied to aparticular system and as embodying various devices diagrammaticallyindicated,

changes and modifications will be obvious to those skilled in the art,and I therefore aim in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is: x

1. In an electric drive, an electric motor, a synchronous generatorsupplying current to-said motor, a prime mover driving said generator,means for varying the excitation of said generator, and means forvarying the torque of said prime mover in accordance with therelationship output of said generator and the maximum output obtainabletherefrom with the same field excitation of said enerator.

2. In an electric drive, an electric motor, a synchronous generatorsupplying current to said motor, a prime mover driving said generator,means for varying the excitation of said generator, and means forvarying the torque of said prime mover so as to maintain a predeterminedratiobetween the actual output of saidgenerator and the pull out torquethereof with the same field excitation.

3. In an electric drive, an electric motor, a plurality of parallelconnected synchronous generators supplying current to said motor, aseparate prime mover driving each generator, means for simultaneouslyvarying the excitation of said generators, and means individual to eachgenerator for varying'the torque of the associated prime mover so as tomaintain a predetermined relationship between the actual output of thegenerator and pull out torque thereof obtainable rest of the system.each torque margin relay i4 with the same field excitation of thegenerator.

4. In an electric drive, an electric motor, a plurality of parallelconnected synchronous generators supplying current to said motor, aseparate prime mover driving each generator, means I for simultaneouslyvarying the excitation of said generators, means for controlling thetorque of each prime mover, and individual means for each generatorresponsive to predetermined electricconditions thereof for, controllingthe associated torque controlling means so as to maintain apredetermined relationship between the actual output of the generatorand the pull out torque thereof under the same field excitationcondition of the generator.

5. In an electric drive, a synchronous motor, a plurality of parallelconnected variable speed synchronous generators supplying current tosaid motor, a separate prime mover driving each generator, a commonexciter for said motor and generators, means for varying the voltage ofsaid exciter, and individual means associated with shown and describedmy invention as each generator and responsive to predetermined electricconditions of the associated generator for controlling the torque of theassociated prime mover so as to maintain a predetermined relationshipbetween the actual output of the associated generator and the pull outtorque thereof obtainable with the same field excitation.

6. In an electric drive, a synchronous motor,

a plurality of parallel connected variable speed synchronous generatorssupplying current to said motor, a separate prime mover driving eachgenerator, a common generators, means exciter for said motor and forvarying the voltage or said

